1. Field of the Invention
This invention relates to methods and materials for increasing the antioxidant protection of polyolefin insulation on electrical conductors, such as telephone cables and electric power cables.
2. Description of the Prior Art
To protect polyethylene, polypropylene, or other types of polyolefin insulation on electrical cables from deterioration, it is known in the plastics industry to add antioxidant material to the polyolefin resin during manufacture of the resin. The antioxidant helps prevent deterioration of the polyolefin during high-temperature extrusion of the insulation onto the wire conductor during manufacture of the cable. The antioxidant also helps prevent cracking and peeling of the insulation, and other ill effects, during the service life of the cable. Such deterioration can occur due to long exposure to the atmosphere, particularly at elevated temperatures of 40 degrees C. and above. The most commonly used antioxidant materials include phenolic type antioxidants, which are often combined with a peroxide decomposer (a so-called "synergist"). The antioxidant and the synergist each typically comprise 0.1% by weight of the insulation.
In more recent years, it has been discovered that the copper conductor in many cables has a catalytic effect that promotes the depletion of the antioxidant. To counter this effect, copper deactivators are now also frequently added to the polyolefin resin during manufacture. Also, the color pigments in many cables have a catalytic effect that promotes deterioration of the insulation.
For these and possibly other reasons, the net effect is that many cables, particularly those manufactured before corrective action for the foregoing debilitating effects was initiated, have deteriorated in service due to depletion of the antioxidant at a faster rate than originally anticipated. For example, it has been found that some multi-conductor telephone cables installed up to 1971 in the southern states have shown signs of deterioration earlier than originally anticipated, particularly in above-ground pedestal-type terminating boxes where the temperature often reaches 40 degrees C. The insulation may become so cracked and brittle that replacement of a portion of the cable, or even the entire cable, is required, which is an expensive solution both in terms of labor and materials.
One attempt to correct this problem for cables already in service in the field has been the formulation of a coating material consisting of an antioxidant in a lacquer-based carrier. The antioxidant comprised about 0.5% by weight of the dried coating, and was intended to migrate into the antioxidant depleted insulation, to restore the original antioxidant level. The antioxidant used was 4, 4' thiobis-(6-tertiary-butyl-meta-cresol), otherwise known by the trade name "Santonox R." This was the same antioxidant material originally introduced into the insulation during manufacture at a level of about 0.1%. However, subsequent tests showed that the actual amount of antioxidant migrating into the insulation was much less than the 0.1% originally present, and furthermore, that this particular antioxidant was rapidly depleted so that little additional protection was obtained.
Clearly, what is needed is a coating formulation that can be applied in the field to partially or totally antioxidant depleted cable, that provides a significant increase in service life.